The EU’s chief negotiator has warned there is still a “risk of failure” in the Brexit negotiations as Brussels continues to reject Britain’s proposals on avoiding a hard border in Ireland.

Michel Barnier said on Friday that a quarter of the work needed to complete preparations for the UK to leave the EU next March remained to be done.

“In terms of what has been agreed so far, it’s about 75%,” he told France 2 television.

Even if Britain and the EU were working towards a British exit from the EU taking place in March 2019, this may not happen if outstanding topics such as Ireland were unresolved, he said.

“There are always difficulties and risks of a failure,” said Barnier. Some in Britain want “what the English call cherry-picking”, he said, adding that the reply to that was: “No way.”

After a progress meeting earlier this week, some EU diplomats fear a crisis in Brexit talks if there is no agreement on the Irish border at a crucial summit in June.

“A backstop has to be in the withdrawal agreement,” said one senior EU diplomat. “There can be no kicking this into the long grass.” If the issue was not solved in June “then we have a crisis” the diplomat said.

The UK continues to insist the border issue cannot be resolved without knowing what the future trading relationship will be – and insists the real deadline is October, not June.

However, Ireland’s deputy prime minister, Simon Coveney, said “serious questions” would start to be asked about Britain’s commitment to an open border in Ireland if there were no fresh ideas on the table by the June deadline.

“We need to see progress on the Irish backstop issue by June,” he said. “It doesn’t need to be a final done deal but, if we don’t see real progress by June, people will start to ask some serious questions.”

The proposals on the table centre on technological solutions for the border. But Coveney told reporters in Cork on Friday: “Those proposals on their own won’t be enough.”

He said the Irish border was one of the biggest challenges in negotiations but was also the issue to which there was most political commitment.

Theresa May has said no British prime minister could ever agree to the EU’s “backstop” option for Northern Ireland, which would require the UK to participate in a “common regulatory area” with the EU.

But government sources said that EU27 negotiators dismissed the UK’s two alternative proposals in a progress meeting earlier this week as “magical thinking” – a phrase that had previously emerged in anonymous briefings.

Donald Tusk, the European council president, warned this week that without a solution on the Irish border, the UK risked leaving the EU without a deal or any transition arrangements.

The EU has written an “anti-backsliding clause” into their prospectus for Brexit talks to stress that negotiations would only progress if previous promises were respected in full.

May has promised the EU that there will be no hard border on the island of Ireland, while setting the goals of leaving the EU customs union and avoiding a border in the Irish sea – three objectives widely seen as incompatible.

Negotiations on the Irish question will continue over four rounds of talks in May and June before the crunch EU summit.

Brussels insiders dismissed reports that Barnier had frozen talks on the UK’s future relationship with the EU, pending progress on Ireland.

But many feel future talks could run into the sand without rapid progress on Ireland or a softening of the UK’s red lines. “It is not that we stop, it is that we don’t move forward,” said a senior EU diplomat. “The red lines do not fit together, not least because of the Irish border.”

A No 10 spokesman said on Friday the UK did not recognise claims that its plans for the Irish border had been subjected to a “systematic and forensic annihilation” by EU officials at a meeting this week with Britain’s lead negotiator, Olly Robbins. But government sources did express exasperation at what they regard as the intransigence of the EU27.

Mujtaba Rahman, of the political consultancy Eurasia group, said: “This is the first time in an official meeting the UK’s Mansion House proposals have been systematically shot down by the EU side.”

The EU27 negotiating team are frustrated that little has changed on the British side since last August, when they first rejected the government’s proposals.

Britain is proposing two options. One is a “customs partnership”, under which the UK would collect duties on the EU’s behalf. The second is a “highly streamlined customs arrangement”, making use of technology to avoid cumbersome border checks.

The government hopes one of these solutions will allow it to reject membership of the customs union and single market, instead pledging looser “regulatory equivalence” on both sides of the border, with the option for future “divergence” – something that the EU has rejected as unworkable.

Conservative MPs who want a soft Brexit believe the government may be deliberately presenting proposals it knows will be knocked back by Brussels to help the prime minister break the deadlock in cabinet about how to proceed.

Backers of a customs union believe it is the only way of resolving the border issue, but it would also constrain the UK’s ability to strike distinct trade deals with non-EU countries.

Boris Johnson, the foreign secretary, told the Daily Telegraph on Wednesday: “Without your ability to do things in a different way if you want, and your ability to do free trade deals, there is very little point in Brexit. I think Theresa totally gets that.”

With a debate and vote on the customs union in the House of Commons next week expected to reveal the level of support among MPs, some believe May could yet be forced into conceding on her red line on the issue.